1. Field of the Invention
The present invention is directed to selected novolak resins made from a phenolic mixture consisting of 2,3-dimethylphenol, 2,6-dimethylphenol, and selected para-substituted lower alkyl phenols and phenolic dimers. The present invention is also directed to radiation-sensitive compositions useful as positive-working photoresists containing these novolak resins.
2. Brief Description of the Prior Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. Generally, in these processes, a thin coating or film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates of printed wiring boards. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam, and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate. In some instances, it may be desirable to bake the imaged coated substrate after the imaging step and before the developing step. This bake step is commonly called a post-exposure bake and is used to increase resolution.
There are two types of photoresist compositions-negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative-working resist with a developer solution causes removal of the nonexposed areas of the resist coating and the creation of a negative image in the photoresist coating, and thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become more soluble to the developer solution (e.g., a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer solution causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. This etchant solution or plasma gases etch the portion of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining resist layer after the development step and before the etching step to increase its adhesion to the underlying substrate and its resistance to etching solutions.
Positive-working photoresists are generally prepared by blending a suitable alkali-soluble binder resin (e.g., a phenol-formaldehyde novolak resin) with a photoactive compound (PAC) which converts from being insoluble to soluble in an alkaline aqueous developer solution after exposure to a light or energy source. The most common class of PAC's employed today for positive-working resists are quinone diazide esters of a polyhydroxy compound. Typical novolak resins used today for positive-working resins are made from various mixtures of ortho-cresol, meta-cresol, and para-cresol which are condensed with an aldehyde source (e.g., formaldehyde).
Positive-working photoresist compositions are currently favored over negative-working resists because the former generally have better resolution capabilities and pattern transfer characteristics.
Photoresist resolution is defined as the smallest feature which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many manufacturing applications today, resist resolution on the order of one micron or less is necessary.
In addition, it is generally desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate.
Increased resolution has been noted in positive photoresist systems whose novolaks possess a high degree of ortho-, ortho-bonding. The term ortho-, ortho-bonding is used to refer to the location and positions of attachment of the methylene bridge between phenolic nuclei. Thus, the methylene bridge which connects two phenolic nuclei which is ortho to both phenolic hydroxyl groups is regarded as ortho, ortho.
It is thought that ortho-, ortho-bonding increases the interactions between phenolic hydroxyls in the novolak and the photoactive compound in positive photoresists compared to positive photoresists containing novolaks which lack a high degree of ortho-, ortho-bonding in their micro-structure. Although the exact character of these interactions is speculative, e.g., hydrogen bonding, van der Waals forces, and the like, there is a correlation between increased resolution and contrast observed in these positive resists whose novolaks contain a high degree of ortho-, ortho-bonding compared to positive resists whose novolaks lack this high degree of ortho-, ortho-bonding.
The optimum number of ortho-, ortho-bonds necessary for optimum interaction between the PAC and the novolak not known. However, it is noted that novolak resins which have a very high percentage of ortho-, ortho-bonding (e.g., a very high content of para-cresol in the novolak) appear to result in photoresists having scum (i.e, undesired residues in the exposed and unexposed areas). Accordingly, having the optimum number of ortho, ortho bonds distributed properly may minimize or eliminate this scum problem.
Besides the positioning of the methylene bridge in the novolak resin, it has been found that the presence of certain phenolic dimer, trimer, and lower oligomeric moieties in the novolak resin may also result in scum formation in the formed and developed images. Further, the presence of these dimers and the like also adversely affect the thermal properties of the final resist and adversely affect the dissolution times of the novolak in the developer solution. Accordingly, it is desirable to remove or prevent the formation of such undesirable moieties from the novolak resin solution before adding it into the photoresist composition.
Also, the formation of scum at the bottom of developed images has also been found to be attributed to the formation of azo-coupling products between the PAC molecule and unreacted 2-, 4-, and 6-position of the novolak phenols (i.e., where the 1-position of the phenol ring is the hydroxy position). The remaining 3- and 5- positions are relatively unreactive under both polymerization reaction conditions and azo coupling reaction conditions.
Accordingly, the present invention is directed to substantially eliminating all of the above-noted three causes of scum formation by a combination of several novolak-forming parameters.
First, the need for a sufficient but not excessive amount of ortho-, ortho-bonding is provided for by employing certain proportions of para-substituted difunctional phenolic moieties (e.g., para-cresol 3,4-dimethylphenol or para-cresol dimer) in the phenolic monomeric mixture used to make the novolak resin.
Second, the unwanted dimers, trimers, and lower oligomers are substantially eliminated by using a molar excess of aldehyde source to total phenolic monomers, as well as employing a certain low proportion of a monofunctional phenolic monomers (i.e., 2,6-dimethylphenol) as terminal groups for the novolak resin.
Third, the unwanted azo-coupling reaction can be substantially eliminated by preblocking substantially all of the reactive 2-, 4-, and 6-positions in the novolak resin.
This preblocking can be accomplished by selecting certain proportions of certain monofunctional, difunctional phenolic monomers or dimers. Monofunctional phenolic monomers are compounds which have only one 2-, 4-, or 6-position relative to the hydroxy group open or unsubstituted to reaction with other phenolic moieties during the novolak polymerization reaction with the phenolic moieties. Examples of monofunctional phenolic monomers are 2,4-dimethylphenol (also known as 2,4-xylenol) and 2,6-dimethylphenol (also known as 2,6-xylenol). Each has only one reactive site, namely the 6- and 4-positions, respectively. The difunctional phenolic monomers and dimers are compounds which have a total of two 2-, 4-, and 6-positions relative to hydroxy groups in the molecule available for reaction. An example of a difunctional phenolic monomer is 4-methylphenol (also known as para-cresol). Another example is 3,4-dimethylphenol (also known as 3,4-xylenol). They both have the 2- and 6-positions open to reaction. An example of a difunctional phenolic dimer is ortho-ortho bonded p-cresol dimer or 2,2'-dihydroxy-5,5'-dimethyldiphenyl methane. It has a single open reactive site on each phenolic ring namely one ortho to each hydroxy group.
If a novolak resin is made by condensing an aldehyde source with the correct relative proportion of monofunctional and difunctional phenolic monomers, one would obtain a novolak resin wherein the interior phenolic moieties were difunctional moieties and the terminal phenolic groups were monofunctional in nature. It should be noted that monofunctional moieties must be terminal groups since once the single available reactive site is reacted during the novolak polymerization step, then there is no further reaction site on that monomeric moiety. In contrast, the difunctional phenolic monomers or dimers must be internal moieties on the novolak chain since the two available reactive sites will react with other phenolic moieties.
Accordingly, there is a need to make novolak resins which substantially eliminate the formation of scum in the developed images, yet have sufficient high thermal resistance properties to be useful in positive-working photoresists. The present invention is believed to be a solution to that need.